I am getting a Speckled Sussex, a Buff Orpington, and a Partridge Cochin and I was wondering if they would do well together or fight and peck each other. Those three breeds are supposed to be really calm and docile.
Are you getting them as chicks and raising them together? If so, they should get along ok with only some minor pecking to work out who is going to be the boss (someone will be the top and someone will be at the bottom). It's trickier if you're getting them as adults from various sources.